This invention relates to a surface-energy gradient on a fluid-impervious surface and method of its creation.
Microscopic fluidic devices, ranging from surgical endoscopes and microelectromechanical systems to the commercial ‘lab-on-a-chip’, allow chemical analysis and synthesis on scales unimaginable a few decades ago (Kataoka and Troian, 1999). Advances in microfabrication techniques have led to the ability to manufacture flow channels ranging from a few hundred angstroms to a few hundred microns (Pfahler, et al, 1990). However, due to the microscopic scale of the systems involved, fluid transport and friction losses are problematic. Different methods using temperature, pressure, or electric potential gradients have been developed to transport fluid through these systems. Each of these methods increases the energy required to operate such systems, and none of the methods solve the problem of fluid friction losses.
Friction arises from the adhesive forces between two surfaces in contact In the absence of wear and plastic deformation, as is the case in fluid transport in microscale systems, friction is largely attributable to interfacial effects (Krim, 1996). For laminar flow in channels, fluid friction loss (f) can be estimated asf=16/Re where Re=Reynolds Number (ρ v DC/μ)                ρ=density of fluid        v=fluid velocity        Dc=effective diameter of channel        μ=viscosity of fluid        
Therefore, as Dc begins to approach micron and angstrom dimensions, friction loss increases greatly.
Organic thin films have been used to control friction and wear in a variety of machines. As machines get even smaller, and lubricating films approach the monolayer regime, self-assembled monolayer films show great potential for use in such items. SAM films have been shown to reduce the friction between two surfaces. By changing the energy of the surface, the SAM can prevent a fluid such as water from wetting the surface. A reduction in the attraction between the fluid and the tail group of the SAM will result in a reduction in friction.
Several studies have been conducted on the frictional properties of SAMs. These studies have shown how friction varies depending on the structure and composition of the SAM. Most of these studies used Atomic Force Microscopy (AFM) to measure friction. This measurement is performed by passing the AFM probe tip over the SAM surface. The frictional response of the surface is measured by the AFM as the normal force exerted by the probe is varied. In their 1996 paper, Xiao et al determined how the chain length of the SAM affected friction. Their work with mica surfaces showed that longer-chain SAMs reduce friction the most. Longer chain molecules form films that are typically more densely packed and more crystalline in structure than shorter chain molecules do. The enhanced crystalline structure and better packing provide a lower friction surface (Liu and Evans, 1996). Their work with SAM films on gold surfaces led to the same conclusions regarding chain length and crystalline structure.
The effect of the tail group was then studied. Researchers determined that frictional behavior closely followed the variation of the adhesive properties, meaning low-energy surfaces had the lowest friction while high-energy surfaces such as —NH2 produced higher amounts of friction loss (Tsukruk and Bliznyuk, 1998). Kim et al in 1999 found that among low energy surfaces, those with the smallest head group yielded the surface with the lowest friction. Specifically, CF3-terminated films had three times the friction of CH3-terminated films.
In addition to lowering the friction between two surfaces, SAMs can have a dramatic effect on the ability of a fluid to wet a surface. For instance, CH3-terminated SAMs produce low energy, hydrophobic surfaces that are not wet by water while CO2H-terminated SAMs produce high energy, hydrophilic surfaces that are almost completely wet by water. The contact angle that water forms with a surface is a good indication of the surface's hydrophilicity or hydrophobicity. For instance, water forms a contact angle of 115° with CH3 surfaces while it forms a contact angle of<15° with CO2H surfaces. In general, as the contact angle decreases, water has more affinity for the surface and will more easily wet it (Laibinis et al, 1998).
A system that reduced friction losses and improved fluid transport would have a great benefit.
All US patents and applications and all other published documents mentioned anywhere in this application are incorporated herein by reference in their entirety.
Without limiting the scope of the invention a summary of some of the claimed embodiments of the invention is set forth below. Additional details of the summarized embodiments of the invention and/or additional embodiments of the invention may be found in the Detailed Description of the Invention below.
A brief abstract of the technical disclosure in the specification is provided as well only for the purposes of complying with 37 C.F.R. 1.72. The abstract is not intended to be used for interpreting the scope of the claims.